Just when we thought we had met all the members of Jack’s immediate family, Tuesday’s episode nixed all of that with one jaw-dropping revelation that will forever change our understanding of the Pearson patriarch. In the final minutes of the episode, tellingly titled “Brothers,” it’s revealed that Jack had a brother, Nicky.

The revelation comes after Jack opts not to see his estranged father, Stanley, on his deathbed. “That man’s been dead to me for a long time,” Jack tells his wife, Rebecca, over the phone, choosing instead to stay with his boys, Kevin and Randall, on their camping trip. It’s only after they return home that Jack, unable to sleep, wanders down to the basement -- hit with a sudden urgency to remember the past. Was he desperate to fill his mind with good memories of his father? Not quite.

Jack opens a random filing cabinet drawer and takes out a metal green box filled with letters, dog tags and photographs from his time in the Vietnam War. He flips through a few frayed pictures before settling on one photograph: It’s of a 20-something version of himself in his army fatigues standing in front of a standard army truck. Next to him is a bespectacled man kneeled down mid-laugh. That’s Nicky, Jack’s brother.

Jack in his 20s during the Vietnam War, with brother Nicky next to him.

It all comes together when we flash back to see young Jack, seemingly around 7 or 8 years old (possibly older), waiting patiently in the truck as his dad makes an unexpected pit stop at a bar on the way to a father-son fishing trip. At first, we’re led to believe that it’s just Jack in the car. Waiting. But in the end, we see Nicky -- having woken up from a deep slumber -- appear from the backseat.

What happened to Nicky? We have five theories about what the reveal means and how this affects the Pearson family tree.

1. Jack and Nicky Are Twins. It certainly would bring everything full circle, wouldn’t it? The photograph of the brothers presumes that they both served in the same battalion at the same time. The draft for the Vietnam War was determined by birthdays, so the chances of Jack and Nicky’s separate birth dates -- if they aren’t twins -- getting drawn seem slim. We’d like to think that Jack and Nicky could be twins; it would certainly explain the Pearson family’s history of multiples. [Edit: A post-episode tweet from series star Mandy Moore seems to have dispelled this notion, noting that Nicky is Jack's "younger brother," but we still like this theory!]

2. Nicky Was Drafted and Jack Volunteered. So, what if Jack and Nick aren’t twins? It did appear that Jack had a predisposition to act more like an older brother to his sibling when they were kids. Could Nicky’s birthday been picked for the war and as a result, Jack volunteered to maintain the promise -- “I’m not going anywhere” -- that he made a decade earlier?

3. Nicky Died in the War. Not much has been etched out about Jack’s time in the war, but in the season one finale, “Moonshadow,” we got a glimpse of his post-Vietnam life as a 28-year-old vet. There was no mention of his brother then. It could very well be that Nicky died a good while earlier, during the war, as there’s been hardly any mention or clues of his existence throughout the Big Three’s upbringing. It would explain Jack's insistence that Kevin and Randall bond on their camping trip, because he knows what it's like to have that one person.

4. Jack’s Dad Had Something to Do With It. It could very well be possible that Nicky returned home from war and his father unceremoniously caused a fatal accident. After all, Jack’s father was an alcoholic. Could Jack telling Rebecca that his dad’s been “dead to him” for a long while be a hint at a tragic rift between the two?

5. Nicky Is Still Alive. Yeah, we think it’s a little insane, too. But why else would they keep emphasizing Nicky’s thick-rimmed glasses, if not only to recognize them in the modern day?

What did you think of Tuesday's episode? Did you see Jack's family twist coming? Tell us your thoughts by tweeting Philiana Ng at @insidethetube or @etnow!